Extended Forecast Discussion NWS Weather Prediction Center College Park MD 141 PM EST Tue Nov 19 2024 Valid 12Z Fri Nov 22 2024 - 12Z Tue Nov 26 2024 ***Strong, long-duration atmospheric river to continue for northern California/southwestern Oregon through late week*** ...Overview... A long-duration atmospheric river will affect the West Coast starting in the short range (Wed) and continue into the first part of the medium range (Fri) as upper and surface lows/frontal systems direct a strong moisture plume into northern California and southwestern Oregon. Several inches of rain are likely to cause rising rivers and some local flooding concerns. Meanwhile a deep upper low will drift slowly from the Great Lakes to the Northeast U.S., leading to cooler temperatures and a cold rain/higher elevation snow for those regions. In between, an upper ridge is likely to traverse the interior West and central U.S. before the upper pattern becomes a bit more zonal into early next week. ...Guidance/Predictability Assessment... The models remain in good agreement overall with the pattern, including the large upper/sfc low pressure system across the Eastern U.S. late in the week. A blend of the latest 00Z/06Z deterministic models worked well as a starting point for Fri-Sun. The guidance has also improved with the expected quasi-zonal flow evolution across much of the central U.S., but some differences appear by early next week in the eastern North Pacific into the West and High Plains. The ECMWF ensembles were slower and deeper than the other models/ensembles by next Mon-Tue off the PacNW but the ECMWF AIFS ENS was about as quick as the GEFS/NAEFS while the deterministic GFS/ECMWF/Canadian were quicker. Thus, opted to meet in the middle given the spread and uncertainty in upstream amplitude that could slow the flow, but could also lead to an undercutting kicker in the mid-latitudes to keep the pattern progressive. ...Weather/Hazards Highlights... The prolonged atmospheric river will be ongoing into the start of the period, extending into Friday across portions of northwestern California and into southwestern Oregon. Sustained moisture flux ahead of the upper low and wavy/slow-moving cold front will result in enhanced orographic lift and likely produce some additional 2 to 4 inch rainfall totals by early Saturday. A Slight Risk area is in place for Day 4 across these areas, including the coastal ranges/northern Sierra and extending to the coast. There will likely be some abatement going into Saturday with the front slowly making its way inland across the region. Snow levels will initially be high, but should fall going into the weekend, including much of the Sierra range where heavy snow will be more likely. However, there is a Marginal Risk area for the lower elevations/foothills of the western Sierra where upslope flow may result in some rainfall totals of 1-2 inches on Day 5 (Sat). Impactful weather is also expected for the Northeast U.S. to close out the work week. This will be the result of a large upper low positioned over the region, in combination with a potentially sub-990 mb surface low over New England. Marginally cold air will be sufficient to support some accumulating snow for the higher terrain near the Great Lakes and interior Northeast (Catskills, Poconos, Adirondacks, into New England etc.), and especially for the central Appalachians where some totals in excess of six inches are possible. There will also be some lake effect rain and snow in the wake of this storm system. The strong surface low will result in windy conditions and periods of heavy rain for much of New England through Friday afternoon as the front clears out the coastal areas. Temperatures will turn noticeably colder across the Eastern U.S. in the wake of the strong cold front and the upper low moving in overhead by Thursday, with highs expected to be about 5 to 15 degrees below average to close out the work week. Given widespread cloud cover, overnight lows will be closer to average with less of a diurnal range for many areas. A warming trend is forecast across the Southern Plains going into the weekend as an upper level ridge builds in and promotes a warmer flow from the Gulf of Mexico, with highs reaching into the 80s for the southern half of Texas. Colder air in Canada may try to start inching closer to the border by next week, but the guidance still shows some timing/location/trajectory differences. Regardless, temperatures will likely be below normal across parts of the northern High Plains next week (20s). Fracasso/Hamrick Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw